Deamidation

The reaction

Asparagine and glutamine carry amide side chains. Over time these hydrolyse to aspartate and glutamate respectively, releasing ammonia. The mass increases by approximately one dalton and the molecule gains a negative charge.

Because the charge changes, deamidated species often separate from the parent peptide chromatographically, which is how the process is monitored.

Sequence dependence

Rate depends heavily on the neighbouring residue. Asparagine followed by glycine deamidates far faster than most other combinations, because the small glycine side chain leaves room for the cyclic intermediate the reaction proceeds through.

A sequence containing Asn-Gly therefore has a shorter practical shelf life than an otherwise comparable one, regardless of how well it is stored.

What slows it

Low pH, low temperature, and above all the absence of water. Deamidation is one of the clearest reasons peptides are supplied lyophilized rather than in solution.